Potentiometer



Nov. 6, 1945.

R. K] FRAZIER POTENTIOMETER Filed June 4, 1942 T0 RECEIVER UNDER TEST INVENTOR RADFORD K. FRAZ/ER ATTORNEY eliminated for satisfactory operation.

Patented Nov. 6, 1945 2,388,534 POTENTIOMETER Radford K. Frazier, Baltimore, Md., assignor to f Bendix Aviation Corporation, South Bend, Ind.,

a corporation of Delaware Application June 4, 1942, Serial No; 445,742

11 Claims.

This invention relates to potentiometers and more particularly to potentiometers for use at very high frequencies.

Potentiometers commercially available at this time usually comprise an arcuately shaped resistance element over which a contacting arm is moved by the rotation of 9. centrally located shaft. Connecting terminals are attached to either end of the resistance element and a third connectins terminal establishes contact to the rotatable arm via a flexible brushor its equivalent. The resistance element normally occupies a circular sector of approximately 330 degrees; and the terminals making connection to the element ends are therefore approximately thirty degrees apart, the arm terminal being located centrally in this thirty degree sector.

The circular shaped resistance element may be viewed as one turn of an inductance coil producing the type of magnetic field normally associated with such coils. This field links, not only with the loop formed by the resistance element itself, but also with the loop circuit enclosed by the movable contact arm and the stationary connector to the rotatable am. With direct current applied to the resistance element and the contact arm rotated to one end of the element, it is found that no potential appears between the contact arm terminal and the winding terminal with which the arm is in contact. With high frequencies of the order of 100 megacycles impressed on the resistance element, however, considerable voltage may be measured between the contact arm terminals and the winding terminal, thus making it impossible to obtain zero voltage output from the potentiometer. This isparticularly detrimental in signal generator attenuators for use at the above frequencies and must be I have traced the source of this voltage to the magnetic coupling existing between the resistance element and the area included between the contact arm and the stationary connector establishing connection to the contact arm.

One of the principal objects of the invention is to provide an improved potentiometer for the control of high frequency alternating currents.

Another object of the invention is to provide a new and novel potentiometer in which the output vs. rotation characteristic in the low output position is independent or the applied frequency.

Yet another object of the invention is to provide an improved potentiometer for use with high frequency alternating currents in which the output voltage measured between the connector to the movable arm and one end of the impedance element is substantially zero when the movable arm is in contact with said end of said impedance element.

The above objects and advantages are accomplished by a potentiometer design in which the magnetic linkage between the potentiometer impedance element and the controlled circuit is substantially zero for a predetermined position of the movable potentiometer arm.

Other objects and advantages of the invention will in part be disclosed and in part be obvious when the following specification is read in coniunction with the drawing in which:

Figure 1 is a rear view showing the essential elements of a potentiometer constructed in accordance with the principles of the invention.

Figure 2 is a side view showing additional details of the potentiometer construction of Figure 1.

Figure 3 is a rear view of an alternative form of potentiometer also designed in accordance with the principles of the invention.

Figure 4 shows a typical utilization circuit wherein the advantages of this new potentiometer are of particular importance.

It is to be understood that the drawing is intended to illustrate specific forms of the invention, and are not to comprise a limitation on the content or scope of the invention.

In the drawing, like parts are designated by like reference characters.

Referring to Figure 1, there is shown the circular impedance element I mounted on the base 2 of insulating material by the screws 3 passing through the mounting ears 4 which form a part of the impedance element I. v For frequencies greater than or megacycles per second a strip of copper or other highly conductive material approximately one thirty-second of, an inch thick may be used for element 1 as the reactance of the loop is suflicient to provide the desired impedance. The arm 5 is movable over the element i upon rotation of the shaft 5, to which the arm 5 may be secured by the sweated collar I. Electrical connection to the arm 5 is established through the fixed contact arm 8- which by virtue of its form in the region of the shaft, is maintained in contact with arm 5 under spring tension. It is to be noted that arm 5 is of somewhat unusual form for a purpose to be discussed more fully below. Electrical connections for the energization of the element 1 may be made by two wires slipped through the apertures 9 and I0 and soldered in place. Apertures H in the base 2 are provided for convenience in mounting the assembly.

Turning to Figure 2, the side view of the potentiometer of Figure 1 shows the bushing l5 mounted in the base 2 and secured by the nut 16. The shaft 6 passes through the'bushing' l5 and is kept in place by the O-shaped washer II which is snapped into the groove l3. The collar 1 is sweated on the shaft 6 and secures the arm 5 thereto, contact between the arm 5 and the fixed arm a being maintained by the tension exerted as a result of the deformation of the shaft end linked to the impedance element by the magnetic field linkage, and even with the arm in contact with the impedance element aperture 9, there will still be an output voltage developed between fixed arm 8 and the connection to the aperture 8, the magnitude of this voltage being controlled by the intensity of the current in element i, the frequency of that current, and the area included between the fixed arm 8 and the movable arm 5. By using an arm 5 having a bend of approximately the shape shown, the voltage appearing between the fixed arm 8 and the aperture 8 when the arm 5 is in the position shown as a result of the flux linking with the area I! is made to oppose and cancel the residual voltage introduced by all other undesired coupling in the potentiometer structure. That zero output is thus attained is due to the fact that the net flux linkage between the potentiometer element loop and the output loop is reduced to zero. To the flux linkages normally existing, there are added, when the arm 5 is in engagement with one end of the element i, the fiux linkages of reverse sense occurring through the area l2 enclosed by arm I and the fixed arm 8. The net flux from the current flowing in element A linking the controlled output terminal circuit (arm 8 and terminal aperture 9) is thus brought to zero. A bend in the contact arm 5 or fixed arm 8 so reversely linking the flux A typical signal generator attenuator circuit is shown in Figure 4, with the radio frequency source I9 connected to the potentiometer 28 through the resistor 2|. The ladder attenuator network and selector switch are connected to the terminals 22 and 23 of the potentiometer. With the potentiometer arm rotated to the maximum clockwise position along the impedance element of the potentiometer, viewing the potentiometer from the rear as shown in this figure, no input voltage is delivered to the input of the attenuator network when using a potentiometer of my design, whereas there has always existed a residual voltage where attenuators of the previous design were employed. In the event that coupling exists between the power circuit and the controlled circuit external to the potentiometer, as indicated by the arrow 25, the potentiometer may be designed along the line which I have previously enumerated and this undesiredvoltage cancelled out when the potentiometer arm is in contact with one of the end points of the impedance element. This potentiometer is of great value in any application where it is desired tocontrol the intensity of a high frequency alternating current by the varying of the position of a contact, especially where very small or zero output potential is desired at a limiting position of the potentiometer contact movement. The circuit just described, however, has been included merely as a specific illustration of the application of this potentiometer and it is not intended to limit the application of the invention to this particular class of service.

It will be obvious that many changes and modifications may be made in the invention without departing from the spirit thereof as expressed in the foregoing description and in the appended claims.

40 What I claim is:

from element I may therefore be conveniently and accurately referred to as a de-gaussing bend, and the relative configuration of these two members be referred to as a de-gaussing configuration. In cases where the potentiometer isto be employed in conjunction with a known circuit configuration, the reverse couplingmay be adjusted in the design to also balance out other undesired potentials induced in portions of the controlled circuit external to the structure of the potentiometer proper.

As an alternative design, the arrangement of Figure 3 shows a potentiometer securing the freedom from magnetic coupling between the impedance element and the controlledcircuit in a somewhat different manner. As before, an arm I1 is secured to the rotatable shaft 8 by the collar 1, and the outer end of the arm ll moves over the impedance element i which is secured to the base 2 by the screws 3 passing through the mounting ears 4. Connection to the impedance element is established by soldering wires in the apertures 9 and it. Contact to the movable arm is afforded by the fixed arm i 8, which is roughly 8- shaped in form. Arm i8 is dished in form of a spring washer in the region of the shaft v8 to provide a contact in tension with arm ii. By virtue of the form of the fixed arm it, which has'also the shape of a de-gaussing bend an area i2 is included between the movable and fixed arms in the minimum output position of the movable arm, whose sign of coupling is such as to oppose the residual coupling existing between'the impedance element and the controlled circuit.

1. In a potentiometer, an impedance element, a movable contact member engaging said element, and a current take-off member electrically connected with said contact member, said members having a cooperating configuration enclosing a de-gaussing area when said contact member is in engagement with a predetermined end of said element.

2. In a potentiometer, an impedance element, a movable contact member engaging said element, and a current take-oil member electrically connected with said contact member, one of said members having a lateral bend intermediate its length defining with said other member a' degaussing area when said movable contact member is in engagement with a predetermined end of said impedance element.

3. In a potentiometer, an impedance element, a movable contact member engaging said element, and a current take-oil member electrically connected with and underlying said contact member, said current take-off member having a bend intermediate its length defining with said movable contact member a de-gaussing area when said movable contact member is in engagement with a predetermined end of said impedance element.

4. In a potentiometer, an impedance element occupying an arcuate sector of less than 360, means for making electrical connection to said impedance element at a plurality of points, a movable contact arm engaging said impedance element, and stationary current take-oi? means electrically connected with said contact arm and having a terminal intermediate the extremities stantially radially from said axis of rotation, said contact arm being'provided with a bend in the plane of rotation defining with said current takeofi member a de-gaussing area when said moi?- able. contact arm is inengagement with a predetermined connection point to said impedance element.

6. In an alternating current system, a source of periodic electrical energy, an impedance element connected across said source and forming a loop circuit therewith, a load impedance, means adjustably and galvanically connecting said load impedance to said first impedance element, whereby a second loop circuit is formed by said load impedance and the connecting means associated therewith, said second loop circuit inherently being magnetically coupled to said first loop circuit, and means for introducing magnetic coupling opposing said inherent coupling between said first 100p circuit and said second loop circuit.

7. In an alternating current. system, a source of periodic electrical energy, an impedance element connected across said source and forming a loop circuit therewith, a load impedance, means adjustably and galvanically connecting said load impedance to said first impedance element, whereby a second loop circuit is formed by said load impedance and the connecting means associated therewith, said second loop circuit inherently being coupled to said first loop circuit, and means for introducing magnetic coupling opposing said inherent coupling between said first 7 loop circuit and said'second loop circuit with a magnitude developing a voltage equal to said inherent coupling when said adjustable means is in engagement with a predetermined point on said first mentioned'impedance..

8. In an alternating current system, a source of periodic electrical energy, an impedance element provided with two and terminals connected across said source and forming a loop circuit therewith, a load impedance, means adJustably and galvanically connecting said load impedance to said first impedance element, whereby a second loop circuit is formed by said load impedance and the connecting means associated therewith, said second loop circuit inherently being coupled to said first loop circuit, and means for introducing magnetic coupling opposing said inherent coupling between said first loop circuit and said second loop circuit, with a magnitude developing a voltage equal to said inherent coupling when said adjustable means is in engagement with a predetermined one of said end terminals.

9. In an alternating current system, a source of periodic electrical energy, an impedance element provided with two end terminals connected across said source andiorming a loop circuit therewith, a load impedance, means for connecting one terminal of said load impedance to one of said end terminals of said impedance element, a stationary current carrying member connected to the other terminal of said load impedance, a movable contact member displaceable over the length of said impedance element connected to said stationary current carrying member, the connections to said load impedance forming a second loop circuit receiving energy from said first loop circuit via space coupling addition to the energy transferred through said contact member, said stationary current carrying member and said movable current carrying member having a cooperating configuration when said contact member is in engagement with one of said end terminals forming a third loop circuit magnetically coupled to said first loop circuit with a sense opposing the coupling between said first loop circuit and said second loop circuit.

10, In an alternating current system, a source of periodic electrical energy, an impedance element provided with two end terminals connected across said source and forming a loop circuit therewith, a load impedance, means for connecting one terminal of said load impedance to one of said end terminals of said impedance element, a stationary current carrying member connected to the other terminal of said load impedance, a

terminal of said impedance element connected to said load impedance forming a third loop circuit magnetically coupled to said first loop circuit 7 I with a sense opposing the coupling between said first loop circuit and said second loop circuit. T

11. In an alternating current system, a source of periodic electrical energy, an impedance element provided with two end terminals connected first loop circuit via space coupling in addition.

to the energy transferred through said contact member, said stationary current carrying member and said movable current carrying member having a cooperating configuration'when said contact member is in engagement with that end terminal of said impedance element connected to said load impedance forming a third loop circuit magnetically coupled to said first loop circuit with a sense opposing and equal to the coupling between said first loop circuit and said second loop circuit.

RADFORD K. FRAZER. 

